1. Field of the Invention
The present invention relates to a color organic EL (electro-luminescent) display which is under development as a thin flat panel display and a driving method thereof and, in particular, relates to a color organic EL display and a driving method thereof where improved display quality is promoted.
2. Description of the Related Art
A color organic EL display receives much attention as a color thin flat panel display similar to a liquid crystal display device (LCD) and a plasma display. FIG. 1 illustrates a schematic cross sectional view showing an EL element and the principle of its luminescence.
An EL element has the following constitution. A transparent electrode 122 made of indium tin oxide (ITO) is formed as a positive electrode on a transparent substrate 121 made of glass or film. Further, an organic hole injection layer 123 and an organic luminescence layer 124 are stacked subsequently on the ITO electrode 122 and a metal layer 125 is formed as a negative electrode on these layers. Here, the organic luminescence layer 124 emits luminescence toward the side of the transparent substrate 121, when a voltage is applied between the ITO electrode 122 and the metal electrode 125.
Further, as the constitution of an organic EL element, there are other ones such as a constitution where only an organic luminescence layer is formed between two electrodes, a constitution where an organic hole injection and transportation layer, an organic luminescence layer and an organic electron injection and transportation layer are formed subsequently between two electrodes, and a constitution where an organic hole injection layer, an organic hole transportation layer, an organic luminescence layer, and an organic electron transportation layer are formed subsequently between two electrodes.
As patterns for use in arrangement of sub pixels, there are a stripe pattern, a mosaic pattern and a delta pattern, for example. FIG. 2 is a schematic view of the conventional stripe pattern, FIG. 3 is a schematic view of the conventional delta pattern and FIG. 4 is a schematic view of the conventional mosaic pattern.
In the conventional stripe pattern, as shown in FIG. 2, a plurality of first electrodes 101 extending in the column direction are formed. This first electrode 101 corresponds to the ITO electrode 122. The length of each first electrode 101 in the horizontal direction (the row direction) is about one third of the length of one pixel in the horizontal direction. A plurality of second electrodes 104 extending in the row direction are formed above the first electrode 101 via an organic luminescence layer and the like. The second electrode 104 corresponds to the metal electrode 125. The width of each second electrode 104 is about the same length as the length of one pixel in the vertical direction (the column direction). The number of the first electrodes 101 is the same as the number of displaying columns and the number of the second electrodes 104 is the same as number of displaying rows. Here, luminescent color from the organic luminescence layer formed between the first electrode 101 and the second electrode 104 is unified at every column and repeated as the sequence of red(R) green (G) and blue (B) in the row direction.
In this stripe pattern, a single sub pixel exists at the cross section of the first electrode 101 and the second electrode 104. A single main pixel is composed of three color sub pixels arranged in the row direction.
This stripe pattern is preferable to display an image having many column lines and row lines such as a table since sub pixels having the same luminescent color are continuously arranged in the column direction.
Further, in the conventional delta pattern, as shown in FIG. 3, the first electrodes 111, extending in the column direction, is formed so that the number of the electrodes is one and a half times of the number of displaying columns. Each of the first electrodes 111 has a charge injection portion 112 and a wiring portion 113 which are formed alternatively. The lengths in the horizontal and vertical directions of the charge injection portion 112 are a half of the lengths in the horizontal and vertical directions of the pixel, respectively. The charge injection portion 112 has a regular square shape or a similar one. The length in the vertical direction of the wiring portion 113 is a half of that of a pixel and the length in the horizontal direction is extremely narrow in comparison with that of the charge injection portion 112. The numbers of the charge injection portions 112 and the wiring portions 113 are the same as the number of displaying rows per the single electrode 111. Further, the charge injection portion 112 and the wiring portion 113 are arranged alternatively in the row direction. A plurality of the second electrodes 114 are formed above the first electrode 111 via an organic luminescence layer and the like. The second electrode 114 extends in the row direction. The number of the second electrodes 114 is the same as that of displaying rows. The first electrode 111 corresponds to the ITO electrode 122 and the second electrode 104 corresponds to the metal electrode 125.
In this delta pattern, a single pixel is composed of the charge injection portions 112 for two colors adjacent to each other in the row direction and the charge injection portion 112 for another different luminescent color, which is provided directly above or directly under them. Namely, a single pixel is composed of three charge injection portions 112 which are located at each peak of xe2x80x9cxcex94xe2x80x9d or its reversed shape.
The delta pattern is preferable to display a natural image or a moving image since there is more irregularity of arrangement of sub pixels in comparison with the stripe pattern.
Further, in the conventional mosaic pattern, as shown in FIG. 4, the first electrode 101 and the second electrode 104 are arranged in a manner similar to the stripe pattern in FIG. 2. But, location of luminescent color of an organic luminescence layer formed between two electrodes is shifted every sub pixel at every one row in the regulated direction. Therefore, the same luminescent color can be obtained every three rows if one pays his/her attention to a series of the first electrode 101. In this mosaic pattern, a single pixel is composed of sub pixels for three colors arranged in the row direction.
The mosaic pattern provides advantages of both the stripe pattern and the delta pattern. In Japanese Patent Laid-open Publication Nos. Hei. 7-248482 and Hei. 10-78590, color LCDs having the mosaic pattern are disclosed.
In the conventional stripe pattern, however, there is a problem where display quality is different between a vertical line and a horizontal line when an image except a white line with emitting luminescence having the same intensity from three sub pixels is displayed, because sub pixels for three colors are arranged adjacently in order in the row direction, while sub pixels having the same color are continuously arranged in the column direction. Namely, there is different display quality between a vertical line and a horizontal line since a horizontal line is displayed as a fine dot line in case of displaying other color""s line except white, though both vertical and horizontal lines are displayed as continuous line in case of displaying a white line.
Further, in the conventional delta pattern, a vertical line is displayed in a zigzag fashion contrary to the stripe pattern. This deteriorates the display quality and is not appropriate to display an image including many vertical and horizontal lines such as a table.
Further, in the conventional mosaic pattern, a series of the first electrode 101 includes sub pixels for three colors. Namely, three colors are illuminated by a single first electrode 101. Thus, the order of outputting signal for each color needs to be changed every one displaying row and signal processing is complicated.
Further, a panel is defined into the upper and the lower portions and two driving circuits for driving the first electrodes are prepared for driving these defined portions in case when a display has many pixels located adjacently to each other. In such case, however, where the above patterns are adopted, displayed image shows discontinuity between the upper portion and the lower portion, if characteristics of the above two driving circuits are different from each other. Further, in case of driving a panel with defining into upper and lower portions, a frame memory having relatively large capacity for storing a half of the image data signal is needed and this increases manufacturing cost.
An object of the present invention is to provide a color organic EL display and a driving method thereof where the display quality can be improved without complicating its driving method.
A color organic EL display according to the present invention comprises first electrodes extending in a column direction, second electrodes extending in a row direction, pixels arranged in a row direction and a column direction, and an organic luminescence layer between the first electrode and the second electrode which emits luminescence by applying a voltage to the first and the second electrodes to make an electric current flow therebetween. The number of the first electrodes is twice of the number of displaying columns. The second electrode is provided at every two displaying rows. Each of the pixels consists of three sub pixels for three luminescence colors arranged in order in the row direction. Each of the sub pixels is provided in each of regions where the first electrodes intersect with the second electrodes in a plan view. The sub pixels are arranged while being shifted to each other by a half pitch in the row direction between adjacent two displaying rows. Luminescence color of sub pixels provided for the same first electrode is unified as a single color. The luminescence colors of three adjacent sub pixels arranged at three triangle positions in two adjacent displaying rows are different from each other.
According to the present invention, a driving method for the first electrode can be simplified since luminescent color from a plurality of sub pixels, provided at the same first electrode, is unified as a single color. Further, smooth displaying can be attained even when an image includes a plenty of vertical lines and horizontal lines such as a table, a moving image and a natural image, since the distances between sub pixels of the same color are relatively uniform each other in both the column and row directions. Further, since an image can be displayed by driving two displaying rows adjacent to each other simultaneously, a new driving method which is similar to the driving method where a panel is defined into two portions and these are driven simultaneously can be adopted. Therefore, high speed scanning can be attained even if a panel is not split into upper and lower portions. Also, a continuous image in a panel can be displayed even when two driving circuits are used and characteristics of them are slightly different from each other. Further, the memory capacity for storing image data can be reduced.
If a luminescence prevention film for preventing a generation of luminescence emitted from a region between the wiring portion and the second electrode is formed on the wiring portion, or a shielding layer is provided on this wiring portion so that this layer prevents the luminescence generated from a region between the wiring portion and the second electrode from being leaked toward outside from the first electrode, a preferable image quality can be obtained by blocking displaying at unnecessary portions.
The method of driving any one of the above mentioned color organic EL displays according to the present invention comprises the step of applying voltages to the first and second electrodes by passive matrix way on the premise that the first electrode is a data electrode and the second electrode is a scanning electrode.